I'll assume you mean the hill is 150 m along its slope, since that's the direction the ball rolls.
The distance x it rolls after time t is
x = (2.5 m/s) t + 1/2 (6.3 m/s²) t²
Set x = 150 m and solve for t :
150 m = (2.5 m/s) t + 1/2 (6.3 m/s²) t²
(3.15 m/s²) t² + (2.5 m/s) t - 150 m = 0
t = 1/2 [(-2.5 m/s) + √((2.5 m/s)² - 4 * (3.15 m/s²) * (-150 m))]
(Use the quadratic formula, and take the positive root)
t ≈ 6.52 s
